Spinal disc annulus reconstruction method and spinal disc annulus stent

ABSTRACT

A surgical method of repair and reconstruction of the spinal disc wall (annulus) after surgical invasion or pathologic rupture, incorporating suture closure, or stent insertion and fixation, designed to reduce the failure rate of conventional surgical procedures on the spinal discs. The design of the spinal disc annulus stent allows ingrowth of normal cells of healing in an enhanced fashion strengthening the normal reparative process.

CROSS-REFERENCE TO A RELATED APPLICATION

This application is a continuation of U.S. Ser. No. 09/947,078, filed Sep. 5, 2001, now U.S. Pat. No. 6,592,625; which is a continuation of U.S. Ser. No. 09/484,706, filed Jan. 18, 2000, (now abandoned); which claims the benefit of U.S. Provisional Application No. 60/160,710, filed Oct. 20, 1999.

FIELD OF THE INVENTION

The invention generally relates to a surgical method of intervertebral disc wall reconstruction. The invention also relates to an annular repair device, or stent, for annular disc repair. The effects of said reconstruction are restoration of disc wall integrity and reduction of the failure rate (3–21%) of a common surgical procedure (disc fragment removal or discectomy). This surgical procedure is performed about 390,000 times annually in the United States.

BACKGROUND OF THE INVENTION

The spinal column is formed from a number of vertebrae, which in their normal state are separated from each other by cartilaginous intervertebral discs. The intervertebral disc acts in the spine as a crucial stabilizer, and as a mechanism for force distribution between the vertebral bodies. Without the disc, collapse of the intervertebral space occurs in conjunction with abnormal joint mechanics and premature development of arthritic changes.

The normal intervertebral disc has an outer ligamentous ring called the annulus surrounding the nucleus pulposus. The annulus binds the adjacent vertebrae together and is constituted of collagen fibers that are attached to the vertebrae and cross each other so that half of the individual fibers will tighten as the vertebrae are rotated in either direction, thus resisting twisting or torsional motion. The nucleus pulposus is constituted of loose tissue, having about 85% water content, which moves about during bending from front to back and from side to side.

The aging process contributes to gradual changes in the intervertebral discs. The annulus loses much of its flexibility and resilience, becoming more dense and solid in composition. The aging annulus is also marked by the appearance on propagation of cracks or fissures in the annular wall. Similarly, the nucleus dessicates, increasing viscosity and thus losing its fluidity. In combination, these features of the aged intervertebral discs result in less dynamic stress distribution because of the more viscous nucleus pulposus, and less ability to withstand localized stresses by the annulus fibrosus due to its dessication, loss of flexibility and the presence of fissures. Occasionally fissures may form rents through the annular wall. In these instances, the nucleus pulposus is urged outwardly from the subannular space through a rent, often into the spinal column. Extruded nucleus pulposus can, and often does, mechanically press on the spinal cord or spinal nerve rootlet. This painful condition is clinically referred to as a ruptured or herniated disc.

In the event of annulus rupture, the subannular nucleus pulposus migrates along the path of least resistance forcing the fissure to open further, allowing migration of the nucleus pulposus through the wall of the disc, with resultant nerve compression and leakage of chemicals of inflammation into the space around the adjacent nerve roots supplying the extremities, bladder, bowel and genitalia. The usual effect of nerve compression and inflammation is intolerable back or neck pain, radiating into the extremities, with accompanying numbness, weakness, and in late stages, paralysis and muscle atrophy, and/or bladder and bowel incontinence. Additionally, injury, disease or other degenerative disorders may cause one or more of the intervertebral discs to shrink, collapse, deteriorate or become displaced, herniated, or otherwise damaged and compromised.

The surgical standard of care for treatment of herniated, displaced or ruptured intervertebral discs is fragment removal and nerve decompression without a requirement to reconstruct the annular wall. While results are currently acceptable, they are not optimal. Various authors report 3.1–21% recurrent disc herniation, representing a failure of the primary procedure and requiring re-operation for the same condition. An estimated 10% recurrence rate results in 39,000 re-operations in the United States each year.

An additional method of relieving the symptoms is thermal annuloplasty, involving the heating of sub-annular zones in the non-herniated painful disc, seeking pain relief, but making no claim of reconstruction of the ruptured, discontinuous annulus wall.

There is currently no known method of annulus reconstruction, either primarily or augmented with an annulus stent.

BRIEF SUMMARY OF THE INVENTION

The present invention provides methods and related materials for reconstruction of the disc wall in cases of displaced, herniated, ruptured, or otherwise damaged intervertebral discs. In accordance with the invention, an annulus stent is disclosed for repair of an intervertebral disc annulus, comprising a centralized hub section, said hub section comprising lateral extensions from the hub section.

In an exemplary embodiment, one or more mild biodegradable surgical sutures are placed at about equal distances along the sides of a pathologic aperture in the ruptured disc wall (annulus) or along the sides of a surgical incision in the annular wall, which may be weakened or thinned.

Sutures are then tied in such fashion as to draw together the sides of the aperture, effecting reapproximation or closure of the opening, to enhance natural healing and subsequent reconstruction by natural tissue (fibroblasts) crossing the now surgically narrowed gap in the disc annulus.

A 25–30% reduction in the rate of recurrence of disc nucleus herniation through this aperture, has been achieved using this method.

In another embodiment as depicted in FIG. 7B, the method can be augmented by creating a subannular barrier in and across the aperture by placement of a patch of human muscle fascia (the membrane covering the muscle) or any other autograft, allograft, or xenograft acting as a bridge or a scaffold, providing a platform for traverse of fibroblasts or other normal cells of repair existing in and around the various layers of the disc annulus, prior to closure of the aperture.

A 30–50% reduction in the rate of recurrence of disc herniation has been achieved using the aforementioned fascial augmentation with this embodiment.

Having demonstrated that human muscle fascia is adaptable for annular reconstruction, other biocompatible membranes can be employed as a bridge, stent, patch or barrier to subsequent migration of the disc nucleus through the aperture. Such biocompatible materials may be, for example, medical grade biocompatible fabrics, biodegradable polymeric sheets, or form fitting or non-form fitting fillers for the cavity created by removal of a portion of the disc nucleus pulposus in the course of the disc fragment removal or discectomy. The prosthetic material can be placed in and around the intervertebral space, created by removal of the degenerated disc fragments.

Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate illustrative embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 shows a perspective view of an illustrative embodiment of an annulus stent.

FIG. 2 shows a front view of the annulus stent of FIG. 1.

FIG. 3 shows a side view of the annulus stent of FIG. 1.

FIGS. 4A–4C show a front view of alternative illustrative embodiments of an annulus stent.

FIGS. 5A–5B show the alternative embodiment of a further illustrative embodiment of an annulus stent.

FIGS. 6A–6B show the alternative embodiment of a further illustrative embodiment of an annulus stent.

FIGS. 7A–7B show a primary closure of an opening in the disc annulus.

FIGS. 8A–8B show a primary closure with a stent.

FIG. 9 shows a method of suturing an annulus stent into the disc annulus, utilizing sub-annular fixation points.

FIGS. 10A–10B show a further illustrative embodiment of an annulus stent with flexible bladder being expanded into the disc annulus.

FIGS. 11A–11D show an annulus stent being inserted into the disc annulus.

FIGS. 12A–12B show an annulus stent with a flexible bladder being expanded.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to an illustrative embodiment of the invention, which appears in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

In one embodiment of the present invention, as shown in FIG. 7A, a damaged annulus 42 is repaired by use of surgical sutures 40. One or more surgical sutures 40 are placed at about equal distances along the sides of a pathologic aperture 44 in the annulus 42. Reapproximation or closure of the aperture 44 is accomplished by tying the sutures 40 so that the sides of the aperture 44 are drawn together. The reapproximation or closure of the aperture 44 enhances the natural healing and subsequent reconstruction by the natural tissue (e.g., fibroblasts) crossing the now surgically narrowed gap in the annulus 42. Preferably, the surgical sutures 40 are biodegradable, but permanent non-biodegradable may be utilized.

Additionally, to repair a weakened or thinned wall of a disc annulus 42, a surgical incision is made along the weakened or thinned region of the annulus 42 and one or more surgical sutures 40 can be placed at about equal distances laterally from the incision. Reapproximation or closure of the incision is accomplished by tying the sutures 40 so that the sides of the incision are drawn together. The reapproximation or closure of the incision enhances the natural healing and subsequent reconstruction by the natural tissue crossing the now surgically narrowed gap in the annulus 42. Preferably, the surgical sutures 40 are biodegradable, but permanent non-biodegradable materials may be utilized.

In an alternative embodiment as depicted in FIG. 7B, the method can be augmented by the placement of a patch of human muscle fascia or any other autograft, allograft or xenograft in and across the aperture 44. The patch acts as a bridge in and across the aperture 44, providing a platform for traverse of fibroblasts or other normal cells of repair existing in and around the various layers of the disc annulus 42, prior to closure of the aperture 44.

In a further embodiment, as shown in FIGS. 8A–B a biocompatible membrane can be employed as an annulus stent 10, being placed in and across the aperture 44. The annulus stent 10 acts as a bridge in and across the aperture 44, providing a platform for a traverse of fibroblasts or other normal cells of repair existing in and around the various layers of the disc annulus 42, prior to closure of the aperture 44.

In an illustrative embodiment, as shown in FIGS. 1–3, the annulus stent 10 comprises a centralized vertical extension 12, with an upper section 14 and a lower section 16. The centralized vertical extension 12 can be trapezoid in shape through the width and may be from about 8 mm–12 mm in length.

Additionally, the upper section 14 of the centralized vertical extension 12 may be any number of different shapes, as shown in FIGS. 4A and 4B, with the sides of the upper section 14 being curved or with the upper section 14 being circular in shape. Furthermore, the annulus stent 10 may contain a recess between the upper section 14 and the lower section 16, enabling the annulus stent 10 to form a compatible fit with the edges of the aperture 44.

The upper section 14 of the centralized vertical extension 12 can comprise a slot 18, where the slot 18 forms an orifice through the upper section 14. The slot 18 is positioned within the upper section 14 such that it traverses the upper section's 14 longitudinal axis. The slot 18 is of such a size and shape that sutures, tension bands, staples or any other type of fixation device known in the art may be passed through, to affix the annulus stent 10 to the disc annulus 42.

In an alternative embodiment, the upper section 14 of the centralized vertical extension 12 may be perforated. The perforated upper section 14 contains a plurality of holes that traverse the longitudinal axis of upper section 14. The perforations are of such a size and shape that sutures, tension bands, staples or any other type of fixation device known the art may be passed through, to affix the annulus stent 10 to the disc annulus 42.

The lower section 16 of the centralized vertical extension 12 can comprise a pair of lateral extensions, a left lateral extension 20 and a right lateral extension 22. The lateral extensions 20 and 22 comprise an inside edge 24, an outside edge 26, an upper surface 28, and a lower surface 30. The lateral extensions 20 and 22 can have an essentially constant thickness throughout. The inside edge 24 is attached to and is about the same length as the lower section 16. The outside edge 26 can be about 8 mm–16 mm in length. The inside edge 24 and the lower section 16 meet to form a horizontal plane, essentially perpendicular to the centralized extension 12. The upper surface 28 of the lateral extensions 20 and 22 can form an angle from about 0–60° below the horizontal plane. The width of the annulus stent 10 may be from about 3 mm–5 mm.

Additionally, the upper surface 28 of the lateral extensions 20 and 22 may be barbed for fixation to the inside surface of the disc annulus 42 and to resist expulsion through the aperture 44.

The device as illustrated can have an unfolded configuration having a width dimension and a height dimension in situ, the width being configured to lie substantially parallel to the circumference of the patient's disc annulus and the height configured to lie substantially parallel to the axis of the patient's spine. The width dimension, as shown, can be greater than the height dimension.

In an alternative embodiment, as shown in FIG. 4B, the lateral extensions 20 and 22 have a greater thickness at the inside edge 24 than at the outside edge 26.

In an illustrative embodiment, the annulus stent 10 is a solid unit, formed from one or more of the flexible resilient biocompatible or bioresorbable materials well know in the art.

For example, the annulus stent 10 may be made from:

a porous matrix of mesh of biocompatible and bioresorbable fibers acting as a scaffold to regenerate disc tissue and replace annulus fibrosus as disclosed in, for example, U.S. Pat. No. 5,108,438 (Stone) and U.S. Pat. No. 5,258,043 (Stone), a strong network of inert fibers intermingled with a bioresorbable (or bioabsorbable) material which attracts tissue ingrowth as disclosed in, for example, U.S. Pat. No. 4,904,260 (Ray et al.);

a biodegradable substrate as disclosed in, for example, U.S. Pat. No. 5,964,807 (Gan et al.); or

an expandable polytetrafluoroethylene (ePTFE), as used for conventional vascular grafts, such as those sold by W. L. Gore and Associates, Inc. under the trademarks GORE-TEX and PRECLUDE, or by Impra, Inc. under the trademark IMPRA.

Furthermore, the annulus stent 10, may contain hygroscopic material for a controlled limited expansion of the annulus stent 10 to fill the evacuated disc space cavity.

Additionally, the annulus stent 10 may comprise materials to facilitate regeneration of disc tissue, such as bioactive silica-based materials that assist in regeneration of disc as disclosed in U.S. Pat. No. 5,849,331 (Ducheyne, et al.), or other tissue growth factors well known in the art.

In further embodiments, as shown in FIGS. 5AB–6AB, the left and right lateral extensions 20 and 22 join to form a solid pyramid or cone. Additionally, the left and right lateral extensions 20 and 22 may form a solid trapezoid, wedge, or bullet shape. The solid formation may be a solid biocompatible or bioresorbable flexible material, allowing the lateral extensions 20 and 22 to be compressed for insertion into aperture 44, then to expand conforming to the shape of the annulus' 42 inner wall.

Alternatively, a compressible core may be attached to the lower surface 30 of the lateral extensions 20 and 22, forming a pyramid, cone, trapezoid, wedge, or bullet shape. The compressible core may be made from one of the biocompatible or bioresorbable resilient foams well known in the art. The core can also comprise a fluid-expandable membrane, e.g., a balloon. The compressible core allows the lateral extensions 20 and 22 to be compressed for insertion into aperture 44, then to expand conforming to the shape of the annulus' 42 inner wall and to the cavity created by pathologic extrusion or surgical removal of the disc fragment.

In an illustrative method of use, as shown in FIGS. 11A–D, the lateral extensions 20 and 22 are compressed together for insertion into the aperture 44 of the disc annulus 42. The annulus stent 10 is then inserted into the aperture 44, where the lateral extensions 20, 22 expand. In an expanded configuration, the upper surface 28 can substantially conform to the contour of the inside surface of the disc annulus 42. The upper section 14 is positioned within the aperture 44 so that the annulus stent 10 may be secured to the disc annulus 42, using means well known in the art.

In an alternative method, where the length of the aperture 44 is less than the length of the outside edge 26 of the annulus stent 10, the annulus stent 10 can be inserted laterally into the aperture 44. The lateral extensions 20 and 22 are compressed, and the annulus stent 10 can then be laterally inserted into the aperture 44. The annulus stent 10 can then be rotated inside the disc annulus 42, such that the upper section 14 can be held back through the aperture 44. The lateral extensions 20 and 22 are then allowed to expand, with the upper surface 28 contouring to the inside surface of the disc annulus 42. The upper section 14 can be positioned within, or proximate to, the aperture 44 in the subannular space such that the annulus stent 10 may be secured to the disc annulus, using means well known in the art.

In an alternative method of securing the annulus stent 10 in the aperture 44, as shown in FIG. 9, a first surgical screw 50 and second surgical screw 52, with eyeholes 53 located at the top of the screws 50 and 52, are opposingly inserted into the adjacent vertebrae 54 and 56 below the annulus stent 10. After insertion of the annulus stent 10 into the aperture 44, a suture 40 is passed down through the disc annulus 42, adjacent to the aperture 44, through the eye hole 43 on the first screw 50 then back up through the disc annulus 42 and through the orifice 18 on the annulus stent 10. This is repeated for the second screw 52, after which the suture 40 is secured. One or more surgical sutures 40 are placed at about equal distances along the sides of the aperture 44 in the disc annulus 42. Reapproximation or closure of the aperture 44 is accomplished by tying the sutures 40 in such a fashion that the sides of the aperture 44 are drawn together. The reapproximation or closure of the aperture 44 enhances the natural healing and subsequent reconstruction by the natural tissue crossing the now surgically narrowed gap in the annulus 42. Preferably, the surgical sutures 40 are biodegradable but permanent non-biodegradable forms may be utilized. This method should decrease the strain on the disc annulus 42 adjacent to the aperture 44, precluding the tearing of the sutures through the disc annulus 42.

It is anticipated that fibroblasts will engage the fibers of the polymer or fabric of the intervertebral disc stent 10, forming a strong wall duplicating the currently existing condition of healing seen in the normal reparative process.

In an additional embodiment, as shown in FIGS. 10A–B, a flexible bladder 60 is attached to the lower surface 30 of the annulus stent 10. The flexible bladder 60 comprises an internal cavity 62 surrounded by a membrane 64, where the membrane 64 is made from a thin flexible biocompatible material. The flexible bladder 60 is attached to the lower surface 30 of the annulus stent 10 in an unexpanded condition. The flexible bladder 60 is expanded by injecting a biocompatible fluid or expansive foam, as known in the art, into the internal cavity 62. The exact size of the flexible bladder 60 can be varied for different individuals. The typical size of an adult nucleus is about 2 cm in the semi-minor axis, 4 cm in the semi-major axis, and 1.2 cm in thickness.

In an alternative embodiment, the membrane 64 is made of a semi-permeable biocompatible material.

In an illustrative embodiment, a hydrogel is injected into the internal cavity 62 of the flexible bladder 60. A hydrogel is a substance formed when an organic polymer (natural or synthetic) is cross-linked via, covalent, ionic, or hydrogen bonds to create a three-dimensional open-lattice structure, which entraps water molecules to form a gel. The hydrogel may be used in either the hydrated or dehydrated form.

In a method of use, where the annulus stent 10 has been inserted into the aperture 44, as has been previously described and shown in FIGS. 12 A–B, an injection instrument, as known in the art, such as a syringe, is used to inject the biocompatible fluid or expansive foam into the internal cavity 62 of the flexible bladder 60. The biocompatible fluid or expansive foam is injected through the annulus stent 10 into the internal cavity 62 of the flexible bladder 60. Sufficient material is injected into the internal cavity 62 to expand the flexible bladder 60 to fill the void in the intervertebral disc cavity. The use of the flexible bladder 60 is particularly useful when it is required to remove all or part of the intervertebral disc nucleus.

The surgical repair of an intervertebral disc may require the removal of the entire disc nucleus, being replaced with an implant, or the removal of a portion of the disc nucleus thereby leaving a void in the intervertebral disc cavity. The flexible bladder 60 allows for the removal of only the damaged section of the disc nucleus, with the expanded flexible bladder 60 filling the resultant void in the intervertebral disc cavity. A major advantage of the annulus stent 10 with the flexible bladder 60 is that the incision area in the annulus 42 can be reduced in size, as there is no need for the insertion of an implant into the intervertebral disc cavity.

In an alternative method of use, a dehydrated hydrogel is injected into the internal cavity 62 of the flexible bladder 60. Fluid, from the disc nucleus, passes through the semipermeable membrane 64 hydrating the dehydrated hydrogel. As the hydrogel absorbs the fluid the flexible bladder 60 expands, filling the void in the intervertebral disc cavity.

All patents referred to or cited herein are incorporated by reference in their entirety to the extent they are not inconsistent with the explicit teachings of this specification, including; U.S. Pat. No. 5,108,438 (Stone), U.S. Pat. No. 5,258,043 (Stone), U.S. Pat. No. 4,904,260 (Ray et al.), U.S. Pat. No. 5,964,807 (Gan et al.), U.S. Pat. No. 5,849,331 (Ducheyene et al.), U.S. Pat. No. 5,122,154 (Rhodes), U.S. Pat. No. 5,204,106 (Schepers at al.), U.S. Pat. No. 5,888,220 (Felt et al.) and U.S. Pat. No. 5,376,120 (Sarver et al.).

It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and preview of this application and the scope of the appended claims. 

1. A method for treating an intervertebral disc having an aperture in the annulus fibrosus, wherein the aperture provides a pathway for the migration of intradiscal material from the subannular space, the method comprising the steps of: providing a device having a first folded configuration and second unfolded configuration, wherein said unfolded configuration has a width dimension, a depth dimension, and a height dimension, wherein, in use, said height dimension lies substantially parallel to the axis of a patient's spine, the depth dimension lies substantially perpendicular to the axis of the spine, and the width dimension lies substantially parallel to the circumference of the patient's disc annulus, and wherein said width dimension is greater than said height dimension and said height dimension is greater than said depth dimension; wherein the first folded configuration comprises dimensions permitting said device to pass through the aperture and the second unfolded configuration comprises at least one dimension larger than a dimension of the aperture; inserting the device through the aperture in the first configuration; and causing or allowing the device to obtain said second configuration whereby said width or height dimension at least partially spans said aperture, thereby providing a barrier restricting the migration of intradiscal material through the aperture.
 2. The method of claim 1 wherein the device is comprised at least in part of a shape memory material.
 3. The method of claim 1, wherein the device is comprised at least in part of biocompatible fabric.
 4. The method of claim 1, wherein the device is at least in part bioabsorbable.
 5. The method of claim 1, further comprising the step of treating the annulus tissue surrounding the aperture to reduce the relative motion thereof.
 6. The method of claim 5, further comprising the step of reapproximating the aperture.
 7. The method of claim 1, wherein the device further comprises at least one fixation element for attaching the device to the annulus.
 8. The method of claim 6 wherein the device further comprises at least one element for reapproximating the aperture.
 9. The method of claim 1, wherein the aperture is pathologic.
 10. The method of claim 1, wherein the aperture is formed at least in part by a surgical incision.
 11. A method for treating an intervertebral disc having an aperture in the annulus fibrosus, wherein the method of claim 1 is performed following discectomy.
 12. The method of claim 1, further comprising the step of applying sutures to acutely stabilize said device.
 13. The method of claim 1, wherein said device is comprised at least in part from autograft.
 14. The method of claim 13, wherein said autograft is muscle fascia.
 15. The method of claim 1, wherein said device is comprised at least in part from allograft.
 16. The method of claim 1, wherein said device is comprised at least in part from a biocompatible membrane.
 17. The method of claim 16, wherein said biocompatible membrane is a medical grade biocompatible fabric.
 18. The method of claim 16, wherein said biocompatible membrane is a medical grade polymeric sheet.
 19. The method of claim 1, wherein said device is applied at least in part in said aperture.
 20. The method of claim 19, wherein said device comprises a filler material.
 21. The method of claim 20, wherein said filler is at least in part form-fitting.
 22. The method of claim 20, wherein said filler is at least in part non-form-fitting.
 23. The method of claim 1, further comprising the step of placing sutures at about equal distances along the sides of the aperture.
 24. The method of claim 1, wherein at least a portion of the device comprises a biodegradable or bioresorbable material.
 25. The method of claim 24, wherein the biodegradable or bioresorbable material comprises a fibrous mesh.
 26. The method of claim 24, wherein the biodegradable material comprises a medical grade polymeric sheet.
 27. The method of claim 1, wherein the device is comprised at least in part of biocompatible fibrous mesh.
 28. The method of claim 1, wherein the device is comprised of a material selected from the group consisting of expandable polytetrafluoroethylene (ePTFE), a material to facilitate regeneration of disc tissue, and a hygroscopic material.
 29. The method of claim 28, wherein the material to facilitate tissue growth comprises a bioactive silica based material.
 30. The method of claim 28, wherein the material to facilitate tissue growth comprises a tissue growth factor.
 31. The method of claim 1, wherein the device is comprised at least in part of polymer fibers.
 32. The method of claim 1, wherein at least a portion of the device is porous.
 33. The method of claim 1, wherein at least a portion of the device is non-porous.
 34. The device of claim 1, wherein at least a portion of the device is formed at least in part of xenograft. 